Aminoplast-cured coating compositions containing polymeric polyols are well known and provide many excellent coating properties. They are inexpensive, durable, and attractive. However, it is widely recognized that such coatings, particularly clear coats, have poor resistance to etching by acid due to the vulnerable ether linkages that are formed between the polyol and aminoplast during curing. Because many geographic areas encounter acidic precipitation, acid etch resistance in coatings is becoming an increasingly desirable property, particularly for automotive coatings.
Coating systems of the prior art which are known to be resistant to acid etch include acid-epoxy curable compositions such as those disclosed in U.S. Pat. No. 4,681,811 and compositions containing hydroxyl functional polymers reacted with isocyanates or polyisocyanates to form polyurethanes. The isocyanates are expensive and the toxicity of the isocyanates is an additional drawback.
Other more recent, promising developments in the field of acid etch resistant coatings include aminoplast-carbamate cured coating compositions, such as those disclosed in U.S. Pat. No. 5,814,410. Carbamate functional polymers are less expensive than isocyanates, do not have the same toxicity drawbacks, and yet provide the same durable urethane bonds in the cured film. A number of processes are available for preparing the carbamate functional polymers, including transcarbamoylation of hydroxyl functional polymers as disclosed in U.S. Pat. No. 5,663,244. Transcarbamoylation of a polymeric polyol using a lower alkyl carbamate is a desirable reaction process because of the relatively low cost, simplicity, and raw material availability. However, high volatility of many lower alkyl carbamates can result in loss of the carbamate from the reaction mixture by sublimation or distillation, leading to poor conversion of hydroxyl to carbamate groups and fouling of the overhead components of the reactor system.
It would be desirable to provide a process for preparing carbamate functional polymers or oligomers via transcarbamoylation using lower alkyl carbamates with improved yields and without loss of reactants.